1. Field of the Invention
The invention relates generally to subsea oil and gas exploration, particularly to methods and systems for electromagnetic surveys of subsea formations.
2. Background Art
Over the past several years, marine controlled-source electromagnetic (CSEM) survey has emerged as a useful tool for the delineation of hydrocarbon reservoirs. In this method a deep-towed electric dipole source is used to excite a low-frequency (˜0.1-10 Hz) electromagnetic signal. This signal propagates through the seawater and subsurface and is perturbed by geologic variation to depths of a few kilometers. Spatially-distributed multi-component, seafloor receivers then record this electromagnetic energy and the data are interpreted in terms of resistivity variations. Of paramount significance is the resistive nature of hydrocarbon formations, which can produce detectable seafloor EM signals. Ideally, CSEM surveys consist of data recorded for many source-receiver offsets, several frequencies and at least two receiver components, inline (radial) and broadside (azimuthal) electric fields.
The standard deployment of CSEM involves a high power transmitter towed by a surface vessel. Two electrodes driven by a high AC current compose the transmitter antenna. The electrodes are kept at the same depth, thereby generating a horizontal electric field in the surrounding environment. The antenna, or horizontal dipole source, is typically towed within ˜50 m of the seafloor, along which multi-component electromagnetic (EM) receivers have been deployed, while continually transmitting to produce currents both in the seawater as well as below the seafloor.
In general, the seafloor is more resistive than the seawater, and, therefore, the electromagnetic skin depth is longer in the Earth than in the seawater column. Because of this, electric field recorded by the receivers is dominated by signals that have diffused through the seafloor and therefore contain information on its structure. By studying the variation of the received signal as a function of source receiver separation and geometry, and the frequency of the transmitted signal, a multi-dimensional picture of the sub-surface resistivity distribution can be built up.
An alternative to the conventional horizontal source is the use of a source that can generate a vertical electric field. It has been reported that such a vertical field source can produce useful information when the returned signals picked up by the sea-bottom receivers are analyzed.
While the prior art methods have been shown to provide useful information about the subsea formations, there still exists a need for better electromagnetic sources for use in marine CSEM surveys.